1.xe2x80x94Field of the Invention
This invention generally relates to expandable intravascular prosthetic devices, also known as stents, that are usually implanted in a patient s body lumen such as an artery to maintain the patency of the lumen after balloon angioplasty.
2.xe2x80x94Description of the Prior Art
a) Stents in Clinical Practice
Stents are usually cylindrically shaped devices which function is to maintain patency or to expand a segment of a body lumen such as an artery. They are particularly useful for preventing a torn or injured arterial lining from occluding a fluid passageway, or for supporting a vessel segment in the presence of suboptimal results following balloon angioplasty. Coronary stenting is gaining widespread acceptance in the treatment of atherosclerotic coronary artery disease. Since the first report by Sigwart et al. of the placement of metallic stents in coronary arteries, coronary stenting has been shown to optimize the geometry of the coronary lumen after balloon angioplasty, to reduce procedural complications and the need for urgent coronary bypass surgery (as a bail-out strategy) and to decrease the rate of restenosis. Coronary stenting is considered by many interventional cardiologists as the therapy of choice for venous graft stenoses. This device is also increasing in popularity for de novo native coronary lesions (primary stenting). Two recent randomized clinical trials comparing stenting with standard balloon angioplasty in primary lesions have demonstrated the efficacy of the Palmaz-Schatz(trademark) stent in reducing the rate of angiographically detected restenosis. In one of these trials, there was both angiographic and clinical benefit, as reflected by; a reduction in major clinical endpoints, especially repeated revascularization of the target lesion.
b) Types of Permanent Stents
Self-expanding Stents
The first stent available for clinical use consisted of a stainless-steel alloy with a self-expanding, spring-like mesh design. The stent was maintained in a constrained and elongated conformation at the distal portion of a delivery catheter by an overlying sheath. When released (by withdrawing the overlying sheath), the stent would automatically expand and increase to a final diameter dependent on the size of the stent and the elasticity of the artery wall. This type of stent is known as the Wallstent(trademark). Although very flexible and providing excellent fluid dynamics, this self-expanding stent was found by some investigators to be deficient since, when deployed, it could exert an undue, permanent stress on the vessel wall. Moreover, significant longitudinal shortening is observed during radial self-expansion, which may result in inappropriate stent placement or inadequate dissection coverage.
Balloon-expandable Stents
The limitations encountered with the Wallstent(trademark) lead to the development of various stents which were controllably expandable within a blood vessel. Generally, in these systems (often called xe2x80x9cballoon-expandable stentsxe2x80x9d) the stent, mounted and crimped on a deflated angioplasty balloon, is delivered to the target area of a blood vessel by a catheter system. Once the stent has been properly positioned (under fluoroscopic guidance), the balloon is expanded thereby expanding the stent so that the latter is urged in place against the vessel wall. At this point, the balloon is deflated, withdrawn and subsequently removed.
Balloon-expandable stents which have gained some notoriety are the Palmaz-Schatz(trademark) stent, the ACS Multilink(trademark) stent, the Wiktor(trademark) stent and the Gianturco-Roubin Flex-Stent(trademark). The Palmaz-Schatz(trademark) and the ACS Multilink(trademark) balloon expandable stents share the common design of a carved stainless-steel cylinder. The Palmaz-Schatz(trademark) stent consists of two rigid stainless-steel slotted tubes joined by a single filament to aid in flexibility. The slotted configuration, when balloon-expanded, deploys as a meshwork. However, despite its articulation, the Palmaz-Schatz stent is known to lack flexibility for delivery in tortuous vessel anatomy and for expansion in angulated lesions. The ACS Multilink(trademark) stent is somewhat similar to the latter, but differs in the increased number of bridges interconnecting identical corrugated rings. In comparison with the Palmaz-Schatz(trademark) stent, the ACS Multilink(trademark) stent has a better flexibility, a more operator-friendly delivery system and less longitudinal shortening during radial expansion.
The Wiktor(trademark) stent, and the Gianturco-Roubin Flex-Stent(trademark) share the common design of a monofilament wire wrapped around an angioplasty balloon catheter. Although they differ from each other in their material, coil structure, radiopacity and degree of wall coverage, they both offer a good flexibility for negotiating tortuous vessels and minimal longitudinal shortening during radial expansion. However, the major drawback is related to the hazard of inadvertent unraveling during manipulation and balloon withdrawal, due to their monofilament design. Other reported disadvantages are the relatively large wire spacing and some degree of recoil after expansion, which may be the reasons why restenosis after use of this type of stent is rather frequent.
All of the stents described above share the common design of being monotubular and thus best suited for delivery in the straight segment of a body passageway. These stents are inappropriate for placement in a bifurcation or a passageway having side branches since: a) the risk of closure of the side branch is increased and b) the side branch will be substantially inaccessible.
The presence of a bifurcation or a major side branch is well recognized as a contraindication for conventional stents.
Indeed, the Physician Guide published in support of the Palmaz-Schatz stent states: xe2x80x9c . . . no attempt should be made following placement of a Palmaz-Schatz stent to access a side branch with a guide wire or a balloon, as such attempts may result in additional damage to the target vessel or the stent. Attempts to treat obstructed side branches within stented segments can result in balloon entrapment, necessitating emergency bypass surgery.xe2x80x9d
Thus, it would be desirable to have an expandable, steerable stent that could prevent obstruction and allow access to side branches, particularly in the field of interventional cardiology. It would be also desirable whether such a stent could be easy to install.
The present invention provides a longitudinally flexible permanent intravascular prosthesis, made of a plastically deformable metal alloy, for implantation in a body lumen and expandable from a contracted condition to an expanded condition, comprising: a plurality of adjacent rings independently expandable in the radial direction and interconnecting members between adjacent rings. The stent comprises two types of rings, each formed by a plurality of either hexagonal or inverted hexagonal elements. The two types of rings are arranged alternately in alignment over the longitudinal axis of the stent, so as to limit spacing between rings and to provide sufficient vessel wall coverage. Two interconnecting members are used to join adjacent rings, positioned at 0xc2x0 and 180xc2x0 in the transverse (cross-sectional) axis to provide flexibility between two rings in one plane; the next pair of interconnecting members is shifted 90xc2x0 from the previous one to provide flexibility to the next attached ring in the perpendicular plane. Thus, this alternately interconnecting member disposition provides a relative flexibility along the longitudinal axis in either the contracted or the expanded state. This particular design has also the property to limit longitudinal expansion or contraction during radial deployment.
The stent is to be mounted and crimped over a balloon catheter, delivered in a contracted state within a body lumen such a patient s artery and expanded passively by the radial forces on the inflating balloon catheter.
A generally tubular stent according to the invention is formed from a deformable material and consists of a plurality of ring-shaped elements of the same radius joined together along a common axis, wherein adjacent ring elements each comprise at least two generally circumferential bands such that the ring element extends axially between a first end band and a second end band which are joined together by struts extending between them, with all of the bands having the same generally zigzag shape having at least three apices and three troughs around the circumference of the band, wherein the apices and troughs of adjacent first band of one ring element and second band of adjacent ring element are aligned on a straight line parallel to the axis, the ring elements being joined each by two links arranged generally parallel to the axis, the two links being radially spaced by an angle of about 180xc2x0, the pair of links joined to one end of each ring element being positioned radially at an angle in the range of 60xc2x0 C. to 120xc2x0 with respect to the pair of links joined to the other end of the respective ring elements, and wherein adjacent bands within each ring are arranged with apices of one and troughs of the other coinciding on a line parallel with the axis, the adjacent bands being joined by connectors joining each of the apices of one band to the respective trough (lying on the said line parallel to the axis) of the adjacent band of the ring element, or vice-versa, whereby the connectors and the portion of adjacent bands joined to connectors form a hexagonal shape with all. hexagonal shapes between a pair of adjacent bands having all internal angles less than 180xc2x0 or having two opposite corners (defined by the bands) with angles greater than 180xc2x0 and the remaining corners having angles less than 90xc2x0.
Each ring element preferably consists of a pair of bands joined together by respective connectors. In this embodiment, ring elements formed of bands joined by connectors to make hexagons having all internal angles less than 180xc2x0 preferably alternate with ring elements formed as hexagons including two internal angles greater than 180xc2x0 (each hereinafter referred to as an xe2x80x9cinverted hexagonxe2x80x9d).
Alternatively a ring element may consist of three bands, with two adjacent bands forming regular hexagons with their respective connectors and with the other pair of adjacent bands forming inverted hexagons or with the other pair of adjacent bands forming regular hexagons. Alternatively three bands may be adapted to form with their connectors a series of inverted hexagons between both pairs of bands.
Each ring element may be formed of more than three bands, with any combination of regular hexagon series and inverted hexagon series within each ring element.
It is preferred for a series of regular hexagons around one end of a ring element to be opposed to a series of inverted hexagons at the adjacent end of the adjacent ring element. Where each element consists of a pair of bands, this result in alternating ring elements forming one series of regular hexagons and ring elements forming one series of inverted hexagons.
The stent generally consists of at least two articulated ring elements, preferably at least three articulated elements, for instance five or more ring elements.
The positioning of the links between adjacent pairs of ring elements, that is between 60xc2x0 and 120xc2x0, gives the stent the freedom to bend at any angle between its ends. The ultimate effect is like that of a universal joint. Where there are only three ring elements therefore the angles between adjacent pairs of links should be about 90xc2x0. Where there are four or more ring elements, the angle may be as low as 60xc2x0 or as high as 120xc2x0. Preferably, however, for every adjacent pair of links the angle between them is about 90xc2x0.
Preferably the stent is made from a circular cylindrical tube starting material from which material is removed, for instance by chemical or laser etching. Preferably the material is made from a metal, for instance a metal which can be plastically deformed, preferably by application of mechanical radially outwardly directed force, for instance by the use of a balloon. Alternatively, but less preferably, the stent may be made from shape memory alloy, such that the radius of the stent can be controlled by controlling the temperature of the stents environment.
The preferred embodiment in which each ring element of the stent consists of two bands with connectors joining the bands so as to form a series of regular hexagons or inverted hexagons with the two types of ring elements alternating with one another allows the spacing between ring elements to be minimized and optimizes vessel wall coverage. The provision of links between the rings at 180xc2x0 optimizes longitudinal flexibility when the stent is unexpanded or in the expanded state. The inverted hexagonal elements limit longitudinal contraction during radial deployment.
Therefore the above present generally consists of a longitudinally flexible permanent prosthesis, made of a plastically deformable metal alloy, for implantation in a body lumen and expandable from a contracted condition to an expanded condition, comprising: a plurality of adjacent rings independently expandable in the radial direction and interconnecting members between adjacent rings. The stent comprises two types of rings, each formed by a plurality of either hexagonal or inverted hexagonal elements. The two types of rings are arranged alternately in alignment over the longitudinal axis of the stent, so as to limit spacing between rings and to provide sufficient vessel wall coverage. Two interconnecting members are used to join adjacent rings, positioned at 0xc2x0 and 180xc2x0 in the transverse (cross-sectional) axis to provide flexibility between two rings in one plane; the next pair of interconnecting members is shifted 90xc2x0 from the previous one to provide flexibility to the next attached ring in the perpendicular plane. Thus, this alternately interconnecting member disposition provides a relative flexibility along the longitudinal axis in either the contracted or the expanded state. This particular design has also the property to limit longitudinal expansion or contraction during radial deployment. On the basis of this general stent design, the present invention also relates to the concept of xe2x80x9cdirectional stentingxe2x80x9d, where a lateral opening is created in the, middle of the stent body, so that the lateral opening can be steered and aligned with the ostium of a bifurcation or a side branch, using a specific delivery system. The stent is to be expanded within a body passageway by the balloon catheter included in the delivery system.
Therefore, in accordance with the present invention, there is provided a generally tubular stent which is formed from a deformable material and consists of a plurality of ring-shaped elements of the same radius joined together along a common axis, wherein adjacent ring elements each comprise at least two generally circumferential bands such that the ring element extends axially between a first end band and a second end band which are joined together by struts extending between them, with all of the bands having the same generally zigzag shape having at least three apices and three troughs around the circumference of the band, wherein the apices and troughs of adjacent first band of one ring element and second band of adjacent ring element are aligned on a straight line parallel to the axis, the ring elements being joined each by two links arranged generally parallel to the axis, the two links being radially spaced by an angle of about 180xc2x0, the pair of links joined to one end of each ring element being positioned radially at an angle in the range of 60xc2x0 to 120xc2x0 with respect to the pair of links joined to the other end of the respective ring elements, and wherein adjacent bands within each ring are arranged with apices of one and troughs of the other coinciding on a line parallel with the axis, the adjacent bands being joined by connectors joining each of the apices of one band to the respective trough (lying on the said line parallel to the axis) of the adjacent band of the ring element, or vice-versa, whereby the connectors and the portion of adjacent bands joined to connectors form a hexagonal shape with all hexagonal shapes between a pair of adjacent bands having all internal angles less than 180xc2x0 or having two opposite corners (defined by the bands) with angles greater than 180xc2x0 and the remaining corners having angles less than 90xc2x0.
Also, in accordance with the present invention, there is provided an expandable prosthesis for a body passageway comprising a plurality of first and second ring-shaped members having ends of complementary shape, at least some of said first members longitudinally alternating with said second members, each pair of adjacent ring-shaped members being interconnected, at least one pair of adjacent alternating first and second members being flexibly connected together with at least one connecting member for allowing said adjacent first and second members to be articulated out of coaxial alignment, said first and second members each having a collapsed diameter for permitting delivery thereof in the body passageway and having at least one expanded diameter upon application from the interior of said first and second members of an outwardly directed force for expanding the body passageway.
Further in accordance with the present invention, there is provided an expandable prosthesis for a body passageway comprising a plurality of interconnected ring-shaped members, at least two pairs of adjacent ring-shaped members being flexibly connected together each with at least two diametrically opposed connecting members for allowing said pairs of adjacent members to be articulated out of coaxial alignment, said connecting members between a pair of adjacent ring-shaped members being angularly offset from adjacent connecting members, said ring-shaped members each having a collapsed diameter for permitting delivery thereof in the body passageway and having at least one expanded diameter upon application from the interior of said first and second members of an outwardly directed force for expanding the body passageway.
In a more specific construction, a side opening is defined laterally in at least two adjacent ring-shaped members of said prosthesis for positioning substantially at an ostium of another body passageway which communicates with the body passageway receiving said prosthesis.
Still further in accordance with the present invention, there is provided an expandable prosthesis for a body passageway comprising a plurality of successive tubular members interconnected together, at least one pair of adjacent alternating tubular members being flexibly connected together with at least one connecting member for allowing said adjacent tubular members to be articulated out of coaxial alignment.
Still further in accordance with the present invention, there is provided an expandable prosthesis for a body passageway comprising at least one tubular member, a side opening being defined laterally in said tubular member of said prosthesis for positioning substantially at an ostium of an other body passageway which communicates with the body passageway receiving said prosthesis, said tubular member having a collapsed diameter for permitting delivery thereof in the body passageway and having at least one expanded diameter upon application from the interior of said tubular member of an outwardly directed force for expanding the body passageway.
Still further in accordance with the present invention, there is provided a method of positioning a prosthesis in a first body passageway at an ostium of a second body passageway communicating with said first body passageway, comprising the steps of providing an expandable prosthesis defining a side opening, inserting said prosthesis into said first body passageway, positioning said prosthesis such that said side opening is substantially opposite said ostium, and expanding said prosthesis against inner walls of said first body passageway with said side opening being located at said ostium.